The present invention relates generally to linearizers, and more particularly, to a bipolar transistor-based linearizer with programmable gain and phase response.
Satellites and other spacecraft are in widespread use for various purposes including scientific research and communications. These scientific and communications missions, however, cannot be accurately fulfilled without wireless communication between a ground station and the spacecraft. In many applications, the satellite relies upon a wireless communication to send and receive electronic data to perform attitude and position corrections, diagnostic status checks, communication calculations and other functions. Without accurate wireless communication, proper satellite function is hindered and at times adversely effected.
Linearizers are commonly used to correct for gain compression and phase variation in both traveling-wave tube amplifiers (TWTAs) and solid state power amplifiers (SSPAs) to obtain improved linearity. Currently, TWTAs and SSPAs suffer a xe2x80x98softxe2x80x99 or xe2x80x98prematurexe2x80x99 clipping of the output power. As the output power deviates from the linear region the TWTA and SSPA introduce distortions that are unacceptable for communications. The function of a linearizer is to minimize this xe2x80x98softxe2x80x99 clipping and make the output power of a TWTA or SSPA as close to xe2x80x98idealxe2x80x99 as possible.
Linearizers are critical in maximizing the revenue from communications satellites by reducing capacity-limiting nonlinearity in transmission channels. They are necessary additions to each TWTA or SSPA on almost every communications satellite and represent a significant fraction of the payload cost. Accordingly, improved linearizer circuits have been aggressively sought after by nearly all producers of communications satellites.
Prior art linearizers have primarily relied on diodes and field effect transistors (FETs) as the nonlinear element within a mechanically tuned circuit to simultaneously match both gain and phase responses. Diode-based linearizers are hybrid modules with many separate parts that need to be individually soldered. Unfortunately, this results in a large and heavy linearizer with less than desirable electrical performance. Additionally, to achieve simultaneous gain and phase match, excessive amounts of manual tuning is required, and once the tuning process is completed, the linearizer""s characteristics are fixed permanently.
The disadvantages associated with these conventional linearizer techniques have made it apparent that a new type of linearizer is needed. Preferably, the new linearizer would allow the gain characteristics to be modified at any time. Additionally, the new linearizer should reduce the size and mass of present linearizers while improving overall electrical performance. The present invention is directed to these ends.
It is, therefore, an object of this invention to provide an improved and reliable bipolar transistor-based linearizer with programmable gain and phase response system. Another object of the invention is to allow the gain characteristics to be modified at any time. An additional object of the invention is to reduce the size and mass of present linearizers while improving overall electrical performance.
In accordance with the objects of this invention, a bipolar transistor-based linearizer with programmable gain and phase response system is provided. In one embodiment of the invention, a bipolar transistor-based linearizer with programmable gain and phase response apparatus uses a splitter to separate an incoming RF signal into two equal components: in-phase (I) and quadrature (Q, ninety degrees delayed). The I signal then passes through a first bipolar variable gain amplifier (VGA) while the Q signal passes through a second bipolar VGA. After passing through the first and second VGAs, the amplified signals are combined at the output using a summer to produce a predistorted signal that drives a TWTA. The gains of each VGA are controlled using an RF power detector in conjunction with a bipolar gain/phase slope controller. Each gain can be adjusted separately to produce a large range of linearization characteristics.
The present invention thus achieves an improved bipolar transistor-based linearizer with programmable gain and phase response. The present invention is advantageous in that it is capable of being electronically tuned of a lifecycle to account for tube aging and environmental effects.
Additional advantages and features of the present invention will become apparent from the description that follows and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims taken in conjunction with the accompanying drawings.